Shamans do have a few conjuration powers. Notable ones that occupy a space are Spirit of Earth Arisen (9), and Great Bear Guardian (19). Both of this conjure a spirit that lasts until the end of the encounter, though it's not your "spirit companion" per se.
A hybrid Shaman|Wizard or Shaman|Druid could also put a few more critters on the board.
I made a Druid/Ranger Hybrid with a wolf beast companion. She then took the Shaman MC feat. So with her first level daily power to summon a wolf there can be 4 wolves in the fight. 1) her beast companion 2) Her wolf Spirit companion 3) her Daily power 4) being shapechanged into a wolf.
The hardest part so far is finding powers because alot of the Beast companion powers don't use the beast for the attacks so she has to have a good strength for alot of Beast ranger powers available to her.